Obsessive-compulsive disorder (OCD) is a debilitating disorder that afflicts 1% to 3% of youth worldwide. Previous research suggests that dysfunction of the frontal-striatal-thalamic circuitry (FSTC) underlies OCD symptoms. Our pilot study demonstrates that 15 adolescents with OCD (mostly medicated) in comparison with 12 controls show significantly lower functional connectivity (associated with higher OCD severity) in multiple FSTC segments. To build on these findings, 25 medication-free adolescents with OCD (ages 12-17) and 25 matched healthy controls will be evaluated using resting-state functional magnetic resonance imaging (R-fMRI) ('functional connectivity) and high angular resolution diffusion imaging (HARDI) (structural connectivity) using advanced data acquisition strategies that are being developed at the University of Minnesota for the NIH-funded Human Connectome Project. All OCD and control participants will undergo R-fMRI and HARDI scans at baseline and 12 weeks later. OCD participants will receive 12 weeks of sertraline, a selective serotonin reuptake inhibitor (SSRI), to treat OCD symptoms. This approach will allow us to examine sertraline effects on FSTC in adolescents with OCD. We hope to identify specific connections in the FSTC that normalize with SSRI treatment. We will also investigate the relations between functional and structural connectivity in each segment of FSTC and correlate OCD dimensions and functional connectivity measures.